1. Field of the Invention
This invention relates to treatment of aqueous materials, particularly, wastewater, and wastewater sludges, and is related to single stage and multistage equalization basin-reactor systems treating variable influent flows.
2. Description of the Prior Art
Many water, wastewater, and other treatment systems have variable influent flows and concentrations. Process controls also produce variable flows and concentrations, for example, through recirculation of various process streams, dilution with the treated streams, by feeding the reagents, and by other control actions. The treatment efficiency depends on the ability of the treatment system to handle the variable characteristics of the influent flows. First, flow and concentration variations are reduced in the treatment system via equalization effects, which are related to the flow patterns and degree of mixing in equalization basins and reactors. Second, the concentration variations are also reduced due to the physical, chemical, physical-chemical, or biological matter transformations. High treatment efficiency and stability, and low capital and operating costs can be provided by utilizing appropriate combinations of flow and concentration equalization basins and reactors. Functions of flow and concentration equalization and materials transformation can be performed in separate flow equalization basins, usually, single or multiple tanks with variable volume of the liquid being treated, concentration equalization basins, single or multiple tanks with a variety of flow and mixing patterns, and in reactors of one or another type. Systems with multiple tank are usually arranged into multi-stage systems. The equalization and transformation functions can also be performed in a single flow-through tank.
For example, a single tank system known under trade name Biolac (U.S. Pat. Nos. 4,287,062, 4,448,689, 4,797,212) combines flow equalization and treatment functions is a single aerobic reactor with a clarifier having a floating weir or a floating clarifier for collecting the clarified water. The weir flow is equal to or greater than the average design flow. Accordingly, the single reactor provides complete flow equalization. Such a reactor can be divided into sequential process zones (U.S. Pat. No. 5,472,611). However, with a single level floating weir or clarifier at the effluent discharge, this reactor functions as a single tank flow equalization basin. Single flow-through tanks with a single level discharge line and floating clarifiers or weirs handle flow variations in a small to moderate range well. For example, these tanks are used for equalizing variations of municipal wastewater flows within a day and industrial wastewater flows within a production shift or a day. However, the required volume of these tanks becomes very large when combined storm and municipal, or industrial, flows need to be equalized. In systems with variable flow recirculations, for example, for diluting or neutralizing the influent, the capacity of single level clarifiers installed for handling minimum to average flows is inadequate. Adding more clarifiers discharging at the same level causes problems with water distribution among clarifiers, or among collection means (such as weirs) in these clarifiers.
If another reactor is added downstream of the single tank equalization basin-reactor, the system becomes a multistage reactor. The downstream reactor will be operated at the constant (equalized) flow rate. The volume of the downstream reactor will stay constant, and it cannot be used for flow equalization. Corresponding depths (liquid level) fluctuations in the first reactor must accommodate all equalization requirements, and will be substantial.
An alternative treatment system for highly variable wastewater flows and concentrations is the sequencing batch reactor (SBR). In this system, usually, two or more single-tank reactors are installed and operated in the periodic mode: filling the wastewater influent, treatment, settling of the sludge, and decanting the wastewater effluent. SBR combines the functions of flow equalization, treatment, and sludge separation in one tank. However, in contrast to multistage systems, it is difficult or impossible to accommodate various process stages, like nitrification and denitrification. With influents having variable flows and concentrations, the end of the treatment period, or a functional treatment step (for example, nitrification) in the sequence of operations is difficult to detect. Accordingly, SBRs are often operated on a time clock set based on a worst case scenario. This results in grossly overdesigned SBR reactors.
The main objective of the present invention is to improve single stage and multi-stage treatment systems, by combining equalization and treatment of wastewater in at least two stages of the multiple stage treatment process. Thus improved systems retain all advantages of multistage systems when treating influents with variable flows and compositions and use more than one reactor for flow equalization. Other advantages will become apparent from the ensuing description of the invention.